1. Field of the Invention
The present invention relates to electrochemical plating systems, and specifically addresses improvements over conventional “contact ring” designs.
2. Description of the Related Art
Copper has taken on a significant role in semiconductor integrated circuit (IC) manufacturing because of its low resistivity and the potential for improved electromigration (EM) performance as compared to aluminum. The current standard for copper metallization is electrochemical plating. One typical apparatus used in electroplating operations is a “contact ring”. However, current contact ring designs are not suitable for all applications.
In conventional IC manufacturing processes, the apparatus used to electroplate material onto a substrate typically includes a plating cell 100 as shown in FIG. 1, which is a schematic diagram of a side view of a typical “fountain” type electroplating cell. FIG. 1 shows a support arm 101, which holds a semiconductor substrate 103 in a contact ring (not shown). Substrate 103 is placed face down on the contact ring and electrically connected to a power supply (not shown) via electrical contacts on the contact ring. The substrate 103 is then immersed in an electrolyte 107 for plating. Electroplating occurs primarily on the downward facing surface of the substrate 103. FIG. 1 shows position A (before pivoting), where the substrate 103 is not immersed in the electrolyte 107, and position B (after pivoting), where the substrate is immersed in electrolyte 107. Typically, support arm 101 is used to immerse substrate 103 using a pendulum like motion, where the angled entry of the substrate into the electrolyte serves to minimize the formation of bubbles on the surface of the substrate during immersion. Generally, support arm 101 is also capable of rotating along its longitudinal axis and periodically raising and lowering, so as to improve plating uniformity during electroplating. Plating cell housing 109 contains the flow of electrolyte 107, which flows upward, like a fountain, while the substrate 103, on which the metallization is to take place, is immersed in the electrolyte 107.
A contact ring, as described above, provides mechanical support for a substrate and electrical contacts which connect the substrate to a power supply in order to enable electroplating operations. FIGS. 2(a) and 2(b) are schematic representations of a typical “wet” contact ring design 200. Typically, in this design, the contact ring and the substrate it supports are fully immersed an electrolyte during electroplating. FIG. 2(a) is a cross-sectional view showing a semiconductor substrate 205 resting on a contact 201 supported by a toroidal contact ring base 203. The substrate 205 is held in place by a clamping device (not shown), such as a backside clamp. FIG. 2(b) is a radial view corresponding to the cross-sectional view in FIG. 2(a). Electroplating occurs on the bottom surface 207 of the substrate 205. Note that this design incorporates a very small gap 208 between the bottom surface 207 of substrate 205 and the upper surface of the toroidal contact ring base 203, which makes the trapping of bubbles during the immersion process quite likely, resulting in bubble defects, plating depressions, and plating swirl due to the inhibition of plating underneath the trapped bubbles. Bubble defects occur in areas where a large potential gap between the electrolyte and a wafer surface is created by bubbles in the electrolyte, inhibiting the plating reaction and leading to the formation of no plating zones. Moreover, since the wafer is rotating, bubbles that form will often spiral out away from the point of formation, leaving swirl-shaped plating defects.
FIGS. 2(c) and 2(d) show schematic representations of a conventional “dry” contact ring design 250. In this design, FIG. 2(c) is a cross-sectional view showing a semiconductor substrate 255 resting on a contact 251 supported by a toroidal contact ring base 253. As in FIGS. 2(a) and 2(b) above, a clamping device (not shown), such as a backside clamp, is used to hold the substrate 255 in place. FIG. 2(d) is a radial view corresponding to the cross-sectional view in FIG. 2(c). Additionally, the dry contact ring design 250 incorporates a barrier 257 in order to isolate electrical contacts 251 from the electrolyte. Note, that in a dry contact ring design 250, only the bottom surface 259 of the substrate 255 comes in contact with the electrolyte.
The advantage of a dry contact ring design is that the electrical contacts are protected from the harsh conditions in the electrolyte during plating operations. However, the dry contact ring design actually worsens the problem of bubble trapping when compared to the wet contact ring design because there is no place for trapped bubbles to escape once they have been formed. One additional issue with using the dry contact ring design is that boundary conditions near the barrier 257 cause a localized increased thickness of electroplated material to be formed. This increased thickness at the edges of the electroplated material on the substrate results in a spike-like profile, similar to that illustrated in FIG. 3, which graphs a thickness profile across the diameter of a semiconductor wafer, illustrating the impact of stagnation points due to fluid boundary conditions.
The spikes in thickness have a significant impact during chemical mechanical polishing (CMP) and can result in Cu residues at the edge of the substrate. In order to remove the spikes at the edge of the electroplated material, the material must be over-polished, leading to increased erosion (sheet ρ variation) at the wafer center.
FIGS. 2(e) and 2(f) show schematic representations of a yet another conventional wet contact ring design 275 where the individual electrical contacts 277 are fully exposed to the electrolyte. In this design, each contact 277 is located on a separate support arm 279. A plurality of support arms 279 replace the toroidal ring structure (as illustrated in FIGS. 2(a)–2(d)). As in the other designs discussed above, a substrate 281 rests on contacts 277 and is held in place by clamping means (not shown). Although replacing the toroidal contact ring base with a plurality of support arms 279 addresses to some extent the bubble trap issue, new concerns arise due to the design differences. A first concern is that the robust electrical contact required for uniform distribution of current during electroplating may be hard to achieve due to the relatively weak support structure provided by individual support arms 279. A second concern is that the electrical contacts 277 must withstand greater exposure to the high acidity of the electroplating solution as well as high current/potential. The additional stress and voltage tolerance requirements induce a need for more expensive materials. On the other hand, if cheaper materials are to be used, then new methods and chemistries must be developed to protect the supports and contacts, for example, implementing a deionized (DI) water cleaning system to rinse the contact ring and substrate after plating. However, implementing new methods results in additional hardware/control requirements as well as, potentially, a loss in throughput due to additional processing time.
On a side note, when using a dry contact ring, such as those discussed above in reference to FIGS. 2(c) and 2(d), a post-plating DI rinse is required before the wafer is removed from the wet section of the apparatus, because the electrolyte, if allowed to enter the dry portion of the plating chamber, will result in corrosion of components and create defects in the plated material due to corrosion particles and precipitation of inorganic salts from the electrolyte.
Another common problem that occurs with conventional contact ring designs is that of “trapped” residual electrolyte, which occurs when wafers are electroplated in succession. Typically, when the wafer is removed from the contact ring after electroplating, the contact ring undergoes a “deplating” process (for wet contacts) in order to clean the electrical contacts prior to receiving the next wafer. If any residual electrolyte is left on the contact ring, “scalloping defects” (i.e., areas with a local thickness that is greater than that of surrounding areas and the overall plated thickness across a wafer) can occur. This is so because the residual electrolyte on the contact ring becomes a source of Cu for local plating, as the current/voltage bias is applied to the wafers before entering the electrolyte. Such electroplated defects can lead to topography differences, resulting in erosion and dishing defects after CMP has been completed. FIG. 4(a) is a photograph of a “scalloping” defect, while FIG. 4(b) shows an atomic force microscopy (AFM) scan across the defect, illustrating the ridge visible in the photograph. The black line visible in FIG. 4(a) shows the path traced by the AFM, while the brackets shown in the figures correlate the two figures.
A second, related problem occurs during the transfer stages after plating has been completed. Once the plating is done, the contact ring and wafer are lifted out of the electrolyte and dried by rotating the assembly for a fixed amount of time. In wet contact ring designs incorporating the features shown in FIGS. 2(a) and 2(b), the low clearance between the substrate 205 and the top surface of the toroidal contact ring base 203 causes the electrolyte to concentrate in the gap if the rotation speed is too slow. Residual electrolyte on the contact ring and on the wafer edge causes “electrolyte induced staining”, where the electrolyte significantly oxidizes the surface of the wafer when the assembly is exposed to air during the transfer from the plating cell to subsequent modules. Electrolyte induced staining can result in erosion and dishing defects (similar to those caused by scalloping defects, discussed above) after CMP has been completed.
The foregoing discussion addresses some limitations of conventional contact ring designs, the use of which can result in potentially yield-impacting defects. For these and other reasons, there is a need for new types of contact rings that can reduce the occurrence of the defects discussed above as well as other defects.